Metal nanomaterials, such as gold nanoparticles (Au NPs), exhibit unique localised surface plasmon resonance, which can be exploited for probing biochemical and biophysical phenomena at the nanoscale and molecular level. Furthermore, the ability to control the synthesis and growth of such nanomaterials using organic and biomimetic molecules, such as nucleic acids and small molecules, facilitates deeper understanding of the interactions between biomolecules and nanomaterials. This thesis described the development of various highly sensitive and novel diagnostic platforms for detecting micro-RNA (miRNA), small molecule and protein biomarkers, by utilising the unique plasmonic properties of Au NPs, as well as modulating the morphology and size of various gold nanostructures. Au NP-conjugated nucleic acid probes, together with a poly(ethylene glycol)-functionalised microarray, enabled highly sensitive and multiplexed detection of miRNAs, conveniently under an optical microscope. Also, colorimetric detection of small molecules using the naked eye was achieved via the controlled growth of aptamer-functionalised Au NPs into various distinct nanostructures, which were dependent on aptamer–target interactions and aptamer-mediated NP growth. Lastly, the interactions between small molecules and Au seeds, and the effect on the size and aspect ratios of grown gold nanorods were investigated and elucidated. The size-modulating mechanism was further incorporated in an immunoassay for the sensitive detection of a protein biomarker, enabling its application in clinical diagnostics. The platforms developed in this thesis could serve as a basis for future development of new biosensing strategies that utilise plasmonic nanomaterials.